The Bush administration says last week's six-party talks on North Korea's nuclear program marked a "good beginning" though there is a long way to go to resolve the issue. A senior official said the United States is ready to discuss a "sequence" of steps providing security assurances to Pyongyang if it verifiably ends is nuclear program.

The senior official who briefed reporters here said that, as expected, there were no breakthroughs in the three days of meetings hosted by China.

But he said he believes the talks "laid a groundwork" for possible progress in follow-on discussions, which he said the United States is "quite optimistic" will be held in Beijing well before the end of the year.

The official, who declined to be further identified but was familiar with the details of the talks, gave a mixed assessment of the stance taken by the North Korea team, which was headed by Pyongyang's Vice Foreign Minister Kim Yong Il.

On the one hand, he said the North Koreans "stated a dedication" to a nuclear-free Korean peninsula and told the five other participants that the communist state does not want to have nuclear weapons.

But he said U.S. officials were disappointed with North Korean presentations which he said were "pre-scripted" and unresponsive to comments by the other parties.

And he said at some points, the North Koreans used "troubling and aggressive language," including he said threats to test a nuclear device made at both an informal one-on-one meeting between Mr. Kim and U.S. delegation chief James Kelly last Wednesday and at a plenary session the next day.

The senior official said the threats to detonate a nuclear device or demonstrate the means to deliver one were "very disturbing" and if carried out would "have consequences" on the atmosphere of the talks and possibly in broader ways.

The Bush administration had said in advance of the talks there would be no reward for what it considered North Korean misbehavior in breaking nuclear agreements since last year, and rejected Pyongyang's demand for a non-aggression pact with Washington.

But the senior official said the United States made clear in Beijing that it was ready to "sincerely discuss" Pyongyang's security concerns in the context of nuclear dismantlement. This he said could occur in a "sequence" of de-nuclearization steps coupled with what he termed "corresponding measures" by the United States and other parties.

The official said the Beijing meetings ended without any specific discussion of what such a sequenced arrangement might entail but said that would be among the subjects of any follow-on meeting.

On related issues, the senior official complained that accounts of the Beijing talks carried by Pyongyang's official media did not reflect the actual content of the meetings, and among other things he denied that Mr. Kelly had called for an "unconditional" end to North Korea's nuclear program as the starting point for a settlement process.

He also said the United States made clear it has no intention of "seeking to strangle" North Korea economically and said a U.S.-led program, the Proliferation Security Initiative, aimed at stopping international trafficking in weapons of mass destruction, should not be seen as a specific threat to North Korea.

North Korea is understood to have raised concern in Beijing about exercises under the initiative involving U.S., Australian and other naval forces due to begin in the Coral Sea off Australia next week.

The official said Mr. Kelly stressed that the exercises are being held far from North Korean territory. He said North Korea should have no worry about interference with its legitimate exports, though adding there are "serious questions" whether moving weapons of mass destruction to so-called "rogue states" or terrorist groups is a legitimate form of trade.